Slow but persistent decrease in the performance of automotive catalysts during prolonged use at the prevailing high temperatures is a phenomenon requiring continuous improvement. This is especially pertinent with respect to the regulations requiring a lifetime greater than 100,000 miles at very low emission limits. Also, the on-board monitoring of such a gradual loss of catalyst activity calls for improved catalytic materials. The loss of activity is related to the loss of a property called "oxygen storage" and to the loss of the dispersion of the active noble metals. This invention relates to the improvement of both aforesaid catalyst attributes.
Specifically, the invention teaches the incorporation of one of the possible ingredients of the active part of the catalyst named "washcoat" in a specific way so as to improve the preservation of oxygen storage and active metal dispersion. The aforesaid ingredient is lanthanum oxide, also known as lanthana. It has been known for some time that the dispersion of lanthana on high-surface-area gamma-alumina, the main surface-area-providing ingredient of the washcoat, inhibits the loss of alumina surface area. More recently, it was found that incorporation of lanthana at the surface of the alumina also results in a higher dispersion of ceria under certain conditions and that lanthana inhibits a reaction between ceria and alumina, both of which could improve oxygen storage and its preservation.
Nevertheless, the literature contains evidence that these latter beneficial effects of lanthana on oxygen storage capacity could well be negated by a direct reaction between the lanthana and ceria. Furthermore, there exists evidence in the literature that lanthana reacts directly with rhodium, one of the main noble metals present in current automotive catalysts, in a way that inhibits its activity.
The subject of this invention is based on our unexpected discovery that when lanthana is deposited in a fashion as described in detail hereinafter so as to form a barrier between the alumina and the ceria or mixtures of ceria and praseodymia, and then an active noble metal such as platinum or palladium is subsequently deposited, there is no inhibition of the oxygen storage capacity or noble metal activity, but rather there is improved preservation of both attributes. Certain conditions have been found necessary in order to obtain these results. Mainly, it is important to coat the alumina with a substantially atomic layer thickness of lanthana, i.e., a layer at least one to two atomic layers thick. In addition, when coating the alumina with lanthana, it is critical that calcination of the lanthana precursor not be performed at too high a temperature in order to prevent formation of a bulk phase at this step in the process.